This invention relates to a printing offset blanket. More particularly, it relates to a printing offset blanket having a surface printing layer, which is superior in printability at the time of high-speed printing, particularly paper discharging properties.
Offset blankets to be used for gravure offset printing are normally formed by providing a surface printing layer on a supporting layer which may have a porous compressive layer in the interior thereof, and the surface printing layer is composed of a layer of a rubber having an elastic modulus and an oil resistance, such as acrylonitrile-butadiene rubber (NBR).
However, when high-speed printing is conducted using a normal offset blanket, an adhesion arises between a paper as a material to be printed and the offset blanket, thereby causing a problem that the paper curls and tears. The similar problem arises at the time of printing on a paper having a smooth surface, such as coat paper. Furthermore, when a normal offset blanket is used for a web offset printing press, such a problem arises that a paper is caught with the offset blanket to cause misregistering, or tearing and scumming of the paper, i.e. so-called delamination arises.
These problems are caused on the ground that the offset blanket is not rapidly separated from the paper, i.e. so-called paper discharging properties are inferior. Since such a problem deteriorates a printing precision and a productivity, considerably, it has hitherto been requested to improve the paper discharging properties of the offset blanket.
As the method of improving the paper discharging properties of the offset blanket, for example, there is suggested a method of making the surface of the surface printing layer rough or a method of increasing a hardness of the surface printing layer. However, according to the former method, the shape of a dot to be formed on the offset blanket becomes inferior, which results in deterioration of a reproducibility of the spot. In addition, according to the latter method, an applicability of ink at the solid portion having a 100% dot surface, i.e. a so-called solid applicability, becomes inferior. That is, there is a problem that the printing precision is deteriorated in both methods.
There is also suggested a method of coating the surface of the surface printing layer with polyvinyl chloride, polyvinyl acetate, silicone rubber, etc. This method causes a problem that the number of steps is increased in the production process of the offset blanket.
Furthermore, there is suggested a method of modifying the surface of the surface printing layer by irradiating ultraviolet ray on the surface of the surface printing layer (Japanese Laid-Open Patent Publication No. 51-37706) or subjecting to a chlorination treatment (Japanese Laid-Open Patent Publication No. 47-51729). However, both methods cause a problem that the number of steps is increased in the production process of the offset blanket and a working atmosphere becomes inferior.
On the other hand, a problem that a paper powder is liable to separate from the surface of the paper by contacting with the offset blanket is caused by the fact that the paper discharging properties of the offset blanket is inferior, as described above. Such a paper powder is accumulated on the surface printing layer of the offset blanket by the long-term printing, thereby causing inclusion of the paper powder into ink. As a result, such a printing failure that white spots are formed at the printed portion arises. Since an opportunity to use a regenerated paper having an inferior paper quality has recently been increasing according to energy saving, recycling, etc., the above problem becomes serious. Furthermore, the surface of the blanket must be frequently washed so as to prevent the printing failure due to inclusion of the paper powder and, therefore, it becomes a problem that a printing operation becomes complicated due to the addition of a washing step.